Mid-infrared vertical cavity laser

ABSTRACT

Disclosed is an optically pumped vertical cavity laser structure operating in the mid-infrared region, which has demonstrated room-temperature continuous wave operation. This structure uses a periodic gain active region with type I quantum wells comprised of InGaAsSb, and barrier/cladding regions which provide strong hole confinement and substantial pump absorption. A preferred embodiment includes at least one wafer bonded GaAs-based mirror. Several preferred embodiments also include means for wavelength tuning of mid-IR VCLs as disclosed, including a MEMS-tuning element. This document also includes systems for optical spectroscopy using the VCL as disclosed, including systems for detection concentrations of industrial and environmentally important gases.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/533,501 filed on Jul. 17, 2017. The contents of U.S.Provisional Patent Application 62/533,501 are hereby incorporated byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contract numberDE-AR0000538 awarded by DOE, office of ARPA-E. The Government hascertain rights in this invention.

FIELD OF THE INVENTION

This invention relates generally to mid-infrared semiconductor lasers,and more particularly to tunable mid-infrared semiconductor lasers, andvertical cavity lasers.

BACKGROUND

Achieving room-temperature continuous-wave (RTCW) vertical cavity laser(VCL) operation at wavelengths beyond about 3.0 microns (um) presentssevere challenges. As of June 2017, electrically pumped VCLs (eVCLs)employing type II interband cascade laser (ICL) technology, thoughpromising, had achieved only room temperature pulsed operation above 3.0um. Two state of the art results are described in “Room-temperatureMid-Infrared Interband Cascade Vertical Cavity Surface Emitting Lasers,”by W. W. Bewley et al in Applied Physics Letters 109, 151108 (2016), andin “Room-temperature vertical cavity surface emitting lasers at 4 umwith GaSb-based type II quantum wells,” by G. K. Veerabathran, et al inApplied Physics Letters 110, 071104 (2017). Achieving RTCW operation inICL eVCLs will require further reduction of operating voltages, and/orreduced optical losses.

If alternative type I InGaAsSb quantum wells for eVCLs are employed, adifferent set of challenges emerges. The band line-up of type I quantumwells with either AlGaAsSb or AlInGaAsSb barriers lattice-matched toGaSb leads to increasingly poor hole confinement with increasingwavelength, resulting in reduced material gain and reduced maximumoperating temperature. This challenge is described in “Type I DiodeLasers for Spectral Region Above 3.0 um,” by G. Belenky, et al, IEEEJournal of Selected Topics in Quantum Electronics, vol. 17. No. 5,September/October 2011. This problem is even more severe in VCLs than inedge-emitting lasers, since VCLs have short gain length and generallyworse thermal impedance than edge-emitters. Thus, RTCW operation hasalso not yet been achieved beyond 3.0 um in VCLs employing type Iquantum wells.

From the foregoing, it is clear that what is required is a verticalcavity laser structure operating at a wavelength >3.0 um, which iscapable of room temperature continuous wave operation.

SUMMARY

An embodiment of the present invention describes the first RTCW VCLstructure operating at wavelengths greater than 3.0 um. An embodiment ofthe present invention employs type I compressively strained quantumwells comprising Indium, Arsenic, and Antimony in an optically pumpedstructure to achieve RTCW VCL operation. Ideally the structure employsperiodic gain, by which it is meant a structure in which at least onequantum well is substantially aligned with a peak in the opticalstanding wave. Periodic gain typically includes structures with multiplequantum wells substantially aligned with multiple standing wave peaks.This optically pumped VCL structure offers several advantages. First,optical pumping requires no dopants in the optical cavity, eliminatingfree carrier absorption, which is the primary source of loss in eVCLs.This also enables consideration of a wider range of materials thanavailable to eVCLs, such as materials which cannot be efficiently doped.Secondly, the periodic gain structure, ideal for optical pumping, butdifficult to implement in electrical pumping, maximizes effective gain,since all quantum wells can be positioned very close to a peak in theoptical standing wave. Periodic gain also alleviates strainaccumulation, enabling the use of a larger number of widely separatedhighly compressively strained quantum wells for higher gain thanelectrically pumped type I structures.

An embodiment of the present invention employs a barrier and claddingdesigned to maximize hole confinement in the InGaAsSb quantum wells.This barrier/cladding is AlInGaAsSb/AlAsSb in one preferred embodiment,pure GaSb in another preferred embodiment, and pure AlAsSb in anotherembodiment.

Further embodiments of the present invention include a tuning mechanismintegrated into the laser cavity to shift the lasing wavelength. Thistuning mechanism can employ thermal tuning and alsomicro-electromechanical systems (MEMS) tuning. Tunable VCLs in the 3-5um range have a numerous applications in the detection of a variety ofspecies, particularly in gas detection. An embodiment of the presentinvention includes systems for optical spectroscopy based on the VCLdisclosed in this document. In these systems, tunable VCL emissionhaving a first wavelength dependence interacts with a sample to create atransformed wavelength dependence, which can be related to a property ofthe sample.

One embodiment of the present invention provides an optically pumpedvertical cavity laser (VCL) optically pumped with a pump source at apump wavelength and providing VCL emission at an emission wavelength,said VCL including a first mirror, a second mirror, and a periodic gainactive region, wherein said periodic gain active region includes atleast two type I quantum wells containing Indium, Arsenic, and Antimony,said active region further including a barrier region adjacent to saidtype I quantum wells which is absorbing at said pump wavelength, and acladding region adjacent to said barrier region, which is substantiallytransparent at said pump wavelength.

Another embodiment of the present invention provides a vertical cavitylaser (VCL) optically pumped with a pump source at a pump wavelength,said VCL including a first mirror, a second mirror, and a periodic gainactive region, wherein said periodic gain active region includes atleast two type I quantum wells containing Indium, Arsenic, and Antimony,said active region further including a GaSb barrier region adjacent tosaid type I quantum wells.

Another embodiment of the present invention provides an optically pumpedvertical cavity laser (VCL) optically pumped with a pump source at apump wavelength and providing VCL emission at an emission wavelength,said VCL including a first mirror, a second mirror, and a periodic gainactive region, wherein said periodic gain active region includes atleast two type I quantum wells containing Indium, Arsenic, and Antimony,and at least one of said first and second mirrors comprises GaAs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is band lineup of various materials lattice matched or nearlylattice matched to GaSb, illustrating hole and electron confinementaccording to an embodiment of the present invention.

FIG. 2A shows a laser structure according to a preferred embodiment ofthe present invention, and FIGS. 2B and 2C show the RTCW laser resultsof this embodiment.

FIG. 3 shows a preferred embodiment of the present invention including aMEMS-tunable structure.

FIG. 4 shows absorption lines of several industrially andenvironmentally important gases in a range of 3000-3600 nm.

FIG. 5 shows pressure-broadened absorption spectra of several gasesimportant in combustion in a range of 4500-6000 nm.

FIG. 6 shows an optical spectroscopy system according to an embodimentof the present invention capable of measuring gas concentration as afunction of spatial location.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description of illustrative embodiments according to principles ofthe present invention is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments of the inventiondisclosed herein, any reference to direction or orientation is merelyintended for convenience of description and is not intended in any wayto limit the scope of the present invention. Relative terms such as“lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,”“down,” “top” and “bottom” as well as derivative thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) should be construed torefer to the orientation as then described or as shown in the drawingunder discussion. These relative terms are for convenience ofdescription only and do not require that the apparatus be constructed oroperated in a particular orientation unless explicitly indicated assuch. Terms such as “attached,” “affixed,” “connected,” “coupled,”“interconnected,” and similar refer to a relationship wherein structuresare secured or attached to one another either directly or indirectlythrough intervening structures, as well as both movable or rigidattachments or relationships, unless expressly described otherwise.Moreover, the features and benefits of the invention are illustrated byreference to the exemplified embodiments. Accordingly, the inventionexpressly should not be limited to such exemplary embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features; the scope of theinvention being defined by the claims appended hereto.

This disclosure describes the best mode or modes of practicing theinvention as presently contemplated. This description is not intended tobe understood in a limiting sense, but provides an example of theinvention presented solely for illustrative purposes by reference to theaccompanying drawings to advise one of ordinary skill in the art of theadvantages and construction of the invention. In the various views ofthe drawings, like reference characters designate like or similar parts.

In the preferred embodiment of the present invention, the operatingwavelength of the VCL is 3-5 um. Preferably, 6-12 compressively strained(1-2% strain) InGaAsSb quantum wells are employed, with pairs of quantumwells at 3-6 standing wave peaks in the optical cavity. The InGaAsSbquantum wells are adjacent to a wider bandgap barrier layer, and thebarrier layer is adjacent to a still wider bandgap cladding layer. Inthe preferred embodiment, the barrier layer is quinary AlInGaAsSbsubstantially lattice-matched to GaSb, and the cladding layer is AlAsSbsubstantially lattice-matched to GaSb. The AlInGaAsSb absorbs apreferred pump wavelength in the range of 1.55 um, while AlAsSb istransparent to this pump wavelength and also serves as a hole blockinglayer, as discussed further below. The amount of quinary material ispreferably adjusted to obtain a single-pass absorption efficiency of theactive region of about 40-80% of the pump light, giving efficient use ofpump energy in combination with relatively uniform pumping of quantumwells. Note that although InGaAsSb is the preferred well composition,InAsSb or other compounds may also be employed to obtain longerwavelengths closer to 5 um.

In an alternate preferred embodiment, the InGaAsSb quantum wells areclad by pure GaSb layers, which can provide good hole confinement andthermal conductivity. Achieving this alternate embodiment, however,requires low temperature growth to overcome strain limitations.Alternately the quantum wells can be clad directly by AlAsSb, without anintermediate AlInGaAsSb layer. This implementation is less preferred asthe wide bandgap AlAsSb does not absorb pump wavelengths near 1.55 um,and absorption will occur only in the quantum wells, reducing absorptionefficiency and increasing required threshold power. In yet anotherembodiment, the AlAsSb cladding could be eliminated, leaving only theAlInGaAsSb barrier. This approach has the disadvantage of increasinglypoor hole confinement when moving to wavelengths substantially longerthan about 3.0 um.

In the preferred embodiment, the presence of AlAsSb provides additionalhole confinement, as shown by the band diagram of FIG. 1. Althoughheterostructure band offsets for materials lattice matched or nearlylattice-matched to GaSb are not precisely known, reasonable estimatescan be made based on the literature, from which qualitative conclusionscan be drawn. FIG. 1 shows estimated band offsets between strainedInGaAsSb quantum wells, an adjacent AlInGaAsSb quinary barrier region,and an AlAsSb cladding region. These offsets are estimated from “The 6.1Angstrom family (GaSb, InAs, AlSb) and its heterostructures: a selectivereview,” by H. Kroemer in Physica E, 20 (2004), 196-203, and from “BandParameters for III-V compound semiconductors and their alloys,” by I.Vurgaftmann, et al in Journal of Applied Physics, vol. 80, no. 11, Jun.1, 2001. Despite some uncertainty in band offset values, the primarypoint of FIG. 1 is that the AlAsSb serves as a hole blocking layer,increasing the confinement of holes more than that provided by thequinary AlInGaAsSb barrier alone.

The 1.55 um pump wavelength is preferred, as cost-effective pump lasersare widely available at this wavelength, and in the range of about1.45-1.65 um. This wavelength is also preferred because it will beabsorbed by quinary AlInGaAsSb lattice-matched to GaSb, which is thepreferred barrier material. Alternate pump wavelengths in the ranges ofabout 0.95-1.15 um and 1.7-2.1 um could be used in alternate preferredembodiments. The use of 6-12 quantum wells (approximate width 9 nm) withAlInGaAsSb barriers (50 nm width) and 1.55 um range pumping in thisdesign provides both adequate gain and adequate absorption length,enabling efficient use of available pump power. Note that pumping at alonger wavelength that is transparent to the AlInGaAsSb barrier, such as2.1 um, though having less absorption, does have the advantage of lessheating for a given amount of absorbed power, resulting in higher outputpower when sufficient pump power is available.

In the preferred embodiment, the VCL uses at least one wafer-bondedmirror containing Al(x)Ga(1-x)As, where 0≤x≤1, grown on a GaAssubstrate. GaAs-based mirrors composed of alternating quarter wavelayers of GaAs/AlGaAs are known to have very low mid-infrared loss, asdiscussed in “High performance near- and mid-infrared crystallinecoatings,” by. G. D. Cole et al, Optica vol. 3, issue 6, pp. 647-656(2016). These mirrors also have large refractive index contrast,correspondingly large bandwidth, are transparent to pumpwavelengths >0.9 um, and have excellent thermal conductivity, as iswell-known to those skilled in the art of NIR VCLs. Additionally, thesemirrors can be grown with the requisite large thicknesses and highsurface quality on large 4 to 6-inch substrates, so are commerciallyattractive for volume production of mid-IR VCLs. Alternate preferredembodiments, however, could use either epitaxially grown or wafer-bondedmirrors grown on GaSb substrates, such as alternating layers ofGaSb/AlAsSb, which also provide high refractive index contrast. The GaSbin the mirror, however, would absorb the preferred pump wavelength of1.55 um, reducing pump efficiency as well as increasing free-carrierloss in the mirror as the pump is absorbed and free-carriers aregenerated. As another preferred embodiment, deposited mirrors such asGermanium/Zinc Sulfide (Ge/ZnS), or mirrors employing ZnSe, ThF4, CaF2,or Si could be used on one or both sides of the optical cavity.

FIGS. 2A-2C show a reduction to practice of a preferred embodiment ofthe present invention, demonstrating RTCW operation near 3.349 um. Asseen in FIG. 2A, the active region of the structure in this embodimentuses 5 pairs of compressively strained (1-2%) quantum wells 210 (about 9nm thickness) with an approximate composition ofIn(0.55)Ga(0.45)As(0.26)Sb(0.74) on 5 peaks of a standing wave 270.Quinary barriers 220 (about 50 nm) with an approximate composition ofAl(0.22)Ga(0.46)In(0.32)As(0.3)Sb(0.7) surround the InGaAsSb quantumwells, and absorb 1550 nm (1.55 um) pump radiation. Lattice matchedAlAsSb cladding regions 230, which are transparent to 1550 nm light,provide additional hole confinement. The distance between pairs of wellsis one half wavelength or approximately 485 nm. The laser employs aGaAs/AlGaAs quarter wave wafer-bonded distributed Bragg reflector (DBR)mirror 240, 250 on either side of the active region. An optical pumpbeam at 1.55 um enters through the GaAs substrate 260 and the 3.349 umemission emerges through the opposite side of the structure.

In alternate preferred embodiments described above, the AlInGaAsSb andAlAsSb in FIG. 2A could be replaced by either pure GaSb or pure AlAsSb.This would require either low-temperature growth due to increased strain(pure GaSb) or higher pump power due to reduced absorption (pureAlAsSb).

As shown in FIGS. 2B and 2C, the emission in this structure issingle-wavelength, representing a single transverse and longitudinalmode. FIG. 2B illustrates the trace provided by an FTIR optical spectrumanalyzer (OSA) both above and below threshold, illustrating a clearlasing peak above threshold and only OSA noise below threshold. FIG. 2Cshows thermal tuning of the lasing wavelength as the pump power isvaried. This device employs a shallow annular etch of approximately 20um inner diameter at the wafer-bonded interface, to provide refractiveindex guiding.

The near field spot size needed to achieve efficient single-modeoperation should preferably be in a range of about 8-26 um for emissionin the range of 3.34 um. This lateral beam dimension roughly scales withwavelength, and the ideal single-mode beam size should be in the rangeof about 2.5-7 times the emission wavelength. The lateral mode fielddiameter can be controlled in a manner analogous to NIR VCSELs, usingetched post or oxide confined geometries, as is well-known to thoseskilled in the art of VCSELs.

The structure of FIG. 2A can be fabricated according to an embodiment asfollows. First the GaAs/AlGaAs DBR mirrors are grown on a GaAssubstrate. In addition, the 10QW periodic gain active region isseparately grown on a GaSb substrate, and includes an InAsSb stop-etchlayer to aid subsequent substrate removal. The DBR mirror and activeregion can be joined by plasma-activated low temperature bonding, as iswell-understood by those skilled in the art of wafer bonding. In thisprocess both the GaAs surface and GaSb surface at the bond interface areplasma activated using an oxygen plasma, and two wafers are joined withthe aid of a small amount of H₂O at the bond interface to create anoxide bond. This oxide may be an oxide of gallium, arsenic, indium, orantimony. An interfacial Al₂O₃ or SiO₂ layer can also be introduced atthe interface to increase bond strength. The bond forms over severalhours at room temperature, and subsequent annealing at 100-200° C. canincrease bond strength. An alternate bonding method is to use metalbonding, such as gold-gold bonding with an aperture in the metal toallow for light passage. After bond formation, the GaSb substrate can beremoved using well-known mixtures of HF/CrO₃, stopping on an InAsSbstop-etch layer. A mixture of 2:1 citric acid:hydrogen peroxide can beused to remove the stop etch layer and stop on a GaSb layer. Next, asecond wafer bonding step joins the second GaAs/AlGaAs mirror to theactive region, completing the laser cavity. The GaAs substrateassociated with the second mirror can be removed using 30:1 H₂O₂:NH₄OHetching, stopping on a high aluminum containing AlGaAs layer as astop-etch. This leaves the entire laser cavity on a single GaAssubstrate associated with the bottom mirror.

The use of optical pumping provides other advantages beyond RTCWoperation. As has been demonstrated in NIR VCLs, optically pumped VCLcavities can often achieve wider tuning range than their electricallypumped counterparts. An embodiment of the present invention thereforealso provides for a tuning mechanism accompanying the optically pumpedVCL structure disclosed here. In the preferred embodiment, the VCLcomprises a fixed half-VCL comprising a fixed mirror and the activeregion, and a second movable mirror separated by a variable gap from thefixed half-VCL. The movable mirror is actuated by amicroelectromechanical system (MEMS). We note that the term “gap” isdefined here to be broad enough to mean any gap that contains no solidmaterial, but may contain air, a vacuum, or any number of gases.

This MEMS tuning mechanism can give a tuning range exceeding 10% of thecenter wavelength at NIR, and may provide similar performance at mid-IR.The MEMS tuning mechanism can be configured to operate in a fixedmanner, or for stepwise tuning, or continuous sweeping, or repetitivesweeping with a repetition rate from DC to greater than 1 MHz. FIG. 3illustrates a preferred embodiment of the periodic gain type I opticallypumped MEMS-tunable VCL. As shown, a fixed bottom mirror 310 is composedof GaAs/AlGaAs and joined at a wafer-bonded interface 320 to a type Iperiodic gain active region 330 comprising InGaAsSb quantum wells asdescribed above. A 1550 nm pump beam enters through the GaAs substrate340 and bottom GaAs/AlGaAs mirror 310, and is absorbed in the type Iactive region 330. The top mirror 350 is disposed on a Silicon nitride(SiN) membrane 360, and is composed of deposited materials. The SiNmembrane 360 includes an aperture, such that no SiN is in the opticalpath. In one preferred embodiment, the top mirror 350 includesalternating layers of ZnS and Ge. Alternate preferred materials for thetop mirror include ZnSe, ThF₄, and CaF₂. In yet another alternateembodiment, the top mirror could be GaAs/AlGaAs and the membrane GaAsinstead of silicon nitride. Ideally, the top mirror should have areflectivity in the range of 99% to 99.9%, and a roundtrip loss of lessthan 0.2%. Application of a voltage between a top contact 370 integralwith the flexible membrane and a bottom contact 380 integral with thefixed half-VCL causes contraction of the gap 390 and tuning to shorterwavelengths. During the fabrication, the gap is occupied with asacrificial layer, such as polyimide, silicon, or germanium, as iswell-known to those skilled in the art of MEMS, and the sacrificiallayer is released near the end of the fabrication process to create thegap and suspended structure. The fabrication of the MEMS actuator shownin FIG. 3 is very similar to MEMS actuators in the near-infrared. Keyfabrication steps are well-known to those skilled in the art ofMEMS-VCSELs, and are described for example in chapter 23 of “OpticalCoherence Tomography: Principles and Applications,” by Wolfgang Drexlerand James Fujimoto, 2^(nd) edition, 2015.

Although the MEMS tuning mechanism of FIG. 3 provides a wide tuningrange, alternate tunable structures are possible, such as thermal tuningby an integrated resistive heater, or by variation of pump power, asshown in FIG. 2C. An additional approach is the use of a second mirrorthat is detached from the half-VCL and is either fixed or movabledepending on the tuning requirements. One specific case would be the useof an optical fiber as the second movable mirror, attached to atransducer such as a piezoelectric translator to effect the tuning. Thisoptical fiber would have an appropriate highly reflective opticalcoating on the fiber end face that acts as the movable mirror.

The tunable VCL according to an embodiment of the present invention canbe incorporated into a number of spectroscopic detection systems. Suchsystems can be configured to detect a variety of properties of a liquid,solid, or gas sample. Examples include concentration of environmentallyand industrially important gases such methane, ethane, ammonia, carbondioxide, water vapor, HF vapor, nitrous oxide, acetylene, carbonylsulfide, dimethyl sulfide, hydrogen cyanide, ozone, and carbon monoxide.FIGS. 4 and 5 illustrate the spectral dependence of the absorption ofseveral of these gases, including pressure-broadened spectra of gasesimportant in combustion in FIG. 5. A common way of measuring gasconcentration is to measure the spectral dependence of tuned laseremission transmitted through a gas using an optical detector, andcompare with the spectral dependence of light incident on the gas.Absorption lines will manifest as dips in the transmitted spectrum andthe magnitude of these dips can with appropriate signal processing berelated to the concentration of the gases of interest. Opticalspectroscopy using the VCL of an embodiment of the present invention canmeasure any change in wavelength dependence of optical emission from theVCL after interaction with a sample, including but not limited tochanged spectral dependence of intensity, polarization, phase, or otherparameters, and relate that change to a property of the sample.Interaction with a sample can also take multiple forms, including butnot limited to transmission, reflection, or scattering. In general,tunable emission from a VCL according to an embodiment of the presentinvention will have a first spectral dependence, which will change to atransformed spectral dependence upon interaction with a sample ofinterest. Quantifying the transformed spectral dependence relative tothe first spectral dependence with an optical detector and appropriatesignal processing, well-known to those skilled in the art of opticalspectroscopy, can be used to determine properties of a sample ofinterest. This analysis can be fed back to optimize another system. Forexample a VCL based spectroscopy system according to an embodiment ofthe present invention can monitor gas concentration in a combustionsystem, and feed back to the combustion engine to optimize for examplefuel efficiency.

Optical spectroscopy can be used along with techniques for spatialmapping to quantify properties of a sample as a function of spatiallocation. FIG. 6 shows an example, in which tunable emission from a VCL610 according to an embodiment of the present invention is steeredacross an oil well pad 630 using, for example, conventionalbeam-steering mirrors. The beam traverses the entire oil well pad as itreflects off various retro-reflectors 620 and returns to an opticaldetector that is co-located with the beam-steered tunable VCL. Analyzingthe detected optical power vs. time can quantify the spatialdistribution of methane gas across the oil well pad, and be used toassess methane leaks. This information can be fed back to a shutoffvalve to turn off or alter a gas flow in response to a detected leak.Other applications of the generic configuration of FIG. 6 could includespatial mapping and monitoring of toxic gases in public setting such asstadiums, parks, airports, or in volcanically active areas.

Note that the VCL capable of RTCW operation described by the presentdisclosure may be employed below room temperature and/or in pulsed modedepending on the application. Such a VCL would still fall under thescope of the present invention.

Note that the VCL of FIGS. 2A and 3 can be fabricated in array form tocreate higher power or multi-wavelength arrays, as has previously beendemonstrated in NIR VCSELs.

While the present invention has been described at some length and withsome particularity with respect to the several described embodiments, itis not intended that it should be limited to any such particulars orembodiments or any particular embodiment, but it is to be construed withreferences to the appended claims so as to provide the broadest possibleinterpretation of such claims in view of the prior art and, therefore,to effectively encompass the intended scope of the invention.Furthermore, the foregoing describes the invention in terms ofembodiments foreseen by the inventor for which an enabling descriptionwas available, notwithstanding that insubstantial modifications of theinvention, not presently foreseen, may nonetheless represent equivalentsthereto.

What is claimed is:
 1. An optically pumped vertical cavity laser (VCL)optically pumped with a pump source at a pump wavelength and providingVCL emission at an emission wavelength, said VCL comprising: a firstmirror, a second mirror, and a periodic gain active region, wherein saidperiodic gain active region comprises at least two type I quantum wellscontaining Indium, Arsenic, and Antimony, said active region furthercomprising a barrier region adjacent to said type I quantum wells whichis absorbing at said pump wavelength, and a cladding region adjacent tosaid barrier region, which is substantially transparent at said pumpwavelength.
 2. The VCL of claim 1, wherein said type I quantum wellsfurther contain Gallium.
 3. The VCL of claim 1, wherein said emissionwavelength is in a range of about 3-5 um.
 4. The VCL of claim 1, whereineach of said quantum wells is compressively strained with a strain in arange of about 1-2%.
 5. The VCL of claim 1, wherein said barrier regioncomprises quinary AlInGaAsSb.
 6. The VCL of claim 1, wherein saidcladding region comprises AlAsSb.
 7. The VCL of claim 1, wherein saidpump wavelength falls within one of the list of ranges from about1.45-1.65 um, about 1.7-2.1 um, and about 0.95-1.15 um.
 8. The VCL ofclaim 1, wherein at least one of said first and second mirrors comprisesAl(x)Ga(1-x)As, with 0≤x≤1.
 9. The VCL of claim 1, wherein both of saidfirst and second mirrors comprise Al(x)Ga(1-x)As, with 0≤x≤1.
 10. TheVCL of claim 1, wherein at least one of said first and second mirrorsincludes Gallium and Antimony.
 11. The VCL of claim 1, furthercomprising a wafer-bonded interface between said active region and atleast one of said first and second mirrors.
 12. The VCL of claim 11,wherein said wafer-bonded interface comprises a plasma-activated bondbetween at least one of mirrors and said active region.
 13. The VCL ofclaim 11, further comprising at least one interfacial oxide layer fromthe group including: Al₂O₃, SiO₂, an oxide of Gallium, an oxide ofarsenic, an oxide of Indium, and an oxide of Antimony, at saidwafer-bonded interface.
 14. The VCSEL of claim 11, further comprising anapertured metal layer at said wafer bonded interface.
 15. The VCL ofclaim 1, comprising at least six type I quantum wells.
 16. The VCL ofclaim 1, comprising multiple pairs of quantum wells, wherein each ofsaid multiple pairs is disposed on a unique standing wave peak.
 17. TheVCL of claim 16, comprising exactly ten quantum wells.
 18. The VCL ofclaim 1, further comprising a mechanism for tuning said emissionwavelength to create a tunable emission over a wavelength tuning range.19. The VCL of claim 1, wherein one of said first and second mirrors isdetached from said active region.
 20. The VCL of claim 1, wherein one ofsaid first and second mirrors is integral with an optical fiber.
 21. TheVCL of claim 18, wherein said tuning mechanism comprises thermal tuning.22. The VCL of claim 21, wherein said thermal tuning is accomplished bychanging a pump power of said pump source.
 23. The VCL of claim 18,wherein said tuning mechanism comprises a MEMS tuning structure tochange a dimension of a gap by moving a position of at least one of saidfirst and second mirrors.
 24. The VCL of claim 23, wherein said gap isevacuated to form a vacuum gap.
 25. The VCL of claim 23, wherein saidfirst mirror is a fixed mirror comprising Al(x)Ga(1-x)As, with 0≤x≤1,and said second mirror is a deposited mirror disposed on a flexiblemembrane and separated from said active region by a gap.
 26. The VCL ofclaim 1, wherein a near field spot size of said VCL emission is in arange of about 2.5-7 times said emission wavelength.
 27. The VCL ofclaim 23, wherein the tuning mechanism is designed to operate in one ofthe list of modes including: a fixed position, a stepwise manner, acontinuous sweep manner, and a repetitive sweeping manner with arepetition rate.
 28. The VCL of claim 27, wherein said repetition rateoperates from fixed to high repetition rate in excess of 1 MHz.
 29. Asystem for optical spectroscopy configured to probe an optical propertyof a sample, the system comprising a tunable laser emitting a tunableemission having a first wavelength dependence over a wavelength tuningrange, said tunable emission interacting with said sample to create atransformed wavelength dependence, an optical detector for detectingsaid transformed wavelength dependence, and a means for converting saidtransformed wavelength dependence to a property of said sample, whereinsaid tunable laser is the VCL of claim
 18. 30. The system of claim 29,further comprising a means for determining a spatial dependence of saidproperty.
 31. The system of claim 30, wherein said means comprises abeam steering mechanism to move said tunable emission.
 32. The system ofclaim 29, wherein said sample is a gas.
 33. The system of claim 29,wherein said sample comprises at least one gas from the group including:methane, ethane, ammonia, carbon dioxide, water vapor, HF vapor, nitrousoxide, acetylene, carbonyl sulfide, dimethyl sulfide, hydrogen cyanide,ozone, and carbon monoxide.
 34. The system of claim 29, wherein saidproperty is a concentration of one constituent.
 35. The system of claim34, wherein said property is a gas concentration.
 36. The system ofclaim 35, further comprising a means for determining a spatialdependence of said gas concentration.
 37. The system of claim 35,further comprising a means to shut off a gas flow in response to saidgas concentration.
 38. The system of claim 35, further comprising ameans for feeding back to an engine combustion system to optimize acombustion process.
 39. A system of claim 29, wherein said sample existswithin a civil structure and its environs, wherein said civil structureis at least one from the group including: an airport terminal, a sportsstadium, a park, and a large public space.
 40. A vertical cavity laser(VCL) optically pumped with a pump source at a pump wavelength, said VCLcomprising: a first mirror, a second mirror, and a periodic gain activeregion, wherein said periodic gain active region comprises at least twotype I quantum wells containing Indium, Arsenic, and Antimony, saidactive region further comprising a GaSb barrier region adjacent to saidtype I quantum wells.
 41. The VCL of claim 40, wherein said pumpwavelength is substantially less than a bandgap wavelength of GaSb. 42.The VCL of claim 40, wherein said emission wavelength is in a range ofabout 3-5 um.
 43. An optically pumped vertical cavity laser (VCL)optically pumped with a pump source at a pump wavelength and providingVCL emission at an emission wavelength, said VCL comprising: a firstmirror, a second mirror, and a periodic gain active region, wherein saidperiodic gain active region comprises at least two type I quantum wellscontaining Indium, Arsenic, and Antimony, and at least one of said firstand second mirrors comprises GaAs.
 44. The VCL of claim 43, wherein saidemission wavelength is in a range of about 3-5 um.